The
National Highway Traffic Safety Administration (NHTSA) is the Federal
agency primarily responsible for safety on the nation’s
highways. NHTSA maintains statistics on specific kinds of
accidents, conducts crash tests using vehicles and dummies, and
performs a number of cutting-edge analyses to improve
crash-worthiness of vehicles and occupant protection.
One
of NHTSA’s highest priorities is cutting down passenger
fatalities from single-vehicle rollovers. NHTSA has developed a
simple formula for determining a car’s “stability factor”,
based on the ratio between the vehicle’s center of gravity
height and its "track width" (width between tire
tracks). However, some automotive engineers believe that there
may be more accurate predictors of a given car’s likelihood of
rolling over in “untripped” (driver-induced) rolls.
Ironically, some low-slung cars, once their wheels leave the
ground, can actually be harder to bring under control than many
cars that fall into the “high CG” category.
Working
in collaboration with another accident reconstruction expert,
James Sobek, Mr. Miller has developed a different approach to
the accurate prediction of roll-over susceptibility. The formula
you see above is based not only on the stability factor, but
also on the distribution of a rolling vehicle’s mass when its
center of gravity is directly over the outside wheels still in
contact with the ground. This orientation is called the
“metastability point”, because once the vehicle rolls past
this point, it is impossible for the operator to regain control
without exceptional maneuvering.
The Miller-Sobek model estimates how rapidly a given
vehicle’s angular roll rate is increasing as it reaches the
metastability angle.
In
a preliminary analysis of their model, the Miller-Sobek team
applied their formula to 15 car models for which extensive
rollover accident information is available. The model correctly
predicted the precise order of accident rates, from fewest to
greatest rollovers per mile. Using the same information,
applying the traditional stability factor currently in use did
not.
(See the Accident
Reconstruction page.)
Miller
and Sobek are refining their research, and soon hope to present
NHTSA with their final analyses for all 2500+ vehicles in the
NHTSA database of vehicle models. If the proposed new formula
works with all types and sizes of vehicles, it may help identify
new vehicle models that are at higher risk.
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